Technologies related to ultrashort pulse lasers began to move forward in the latter half of 1960, and have progressed rapidly to be studied actively in recent years. As a light source thereof, an expensive, large, high-precision solid state laser device, typified by a titanium-sapphire laser, is mainly used, and this point is a factor that inhibits widespread use of the technology. If an ultrashort pulse laser can be implemented by a semiconductor laser element, significant size reduction, cost reduction, and high stabilization are expected to be achieved, making a breakthrough in spreading the use of advanced science and technology in this field. For example, if an ultrashort pulse laser whose wavelength region is the 405 nm band can be implemented by only a semiconductor laser element, it can be used as a volumetric optical disk light source in the next generation to Blu-ray (registered trademark), and moreover, a handy ultrashort pulse light source covering the entire wavelength band in the visible light region can be obtained. This makes it possible to provide a light source that is required in a wide variety of fields, including medical fields, bioimaging fields, and optical shaping fields, which presumably contributes greatly to the progress of science and technology.
In addition, an increase in output power is a big problem for a laser light source. Therefore, as well as an increase in output power of a semiconductor laser element, a semiconductor optical amplifier (SOA) has been under study, as a means for amplifying light from a laser light source. Conventionally, optical amplifiers have been developed mainly for optical communication, and therefore practical utilization of a semiconductor optical amplifier in the 405 nm band has hardly been seen. A semiconductor optical amplifier for the 1.5 μm band using a GaInAsP-based compound semiconductor and having a tapered ridge stripe structure is known from JP H5-067845A, for example. The technology disclosed in JP H5-067845A expands a mode field in accordance with an optical waveguide width by gradually increasing the optical waveguide width in a tapered shape from a narrow input-side optical waveguide satisfying a single-mode condition to an output-side optical waveguide in a semiconductor optical amplifier, thus expanding the maximum output of the semiconductor optical amplifier.
In addition, a semiconductor laser element (specifically, a mode-locked semiconductor laser element) that is formed of a stacked structure body of a first compound semiconductor layer, a third compound semiconductor layer, and a second compound semiconductor layer and has a ridge stripe structure is known from JP 2012-151210A. In the semiconductor laser element, the stacked structure body forms three regions (a first light emitting region, a saturable absorption region, and a second light emitting region), and the second light emitting region positioned at the light emitting end surface side has a tapered shape with a width increasing toward the light emitting end surface.